The fee will almost certainly be from their US correspondent bank (the one that sends the money on to the Credit Union) what is surprising is that RBS cannot tell how much that fee is.
Ask to speak to the manager and I'm sure s/he'll be able to tell you the receiving charge amount is, It's standard for all RBS sent to the US, likewise at Barclays we have a standard fee for the US which is different to Canada etc.
Another thing to have a look at is the charging arrangements, we normally offer three types (using above costs)
a UK account to pay all fees (you are charged £1,000+ £20 +$30)
b UK account to pay UK fees (you are charged £1,000 + £20, US account is charged $30)
c UK account pays no fees (you are charged £1,000, US pays £20 + $30)
Have you perhaps recently changed which box you've ticked?
http://www.rbs.co.uk/Personal_Finances/Travel_and_International_Services/Making_International_Payments/roy_econ.htmThe section under charges implies that the other side makes charges, hope this helps